Direct fuel injection (DI) engines provide some advantages over port fuel injection systems. For example, direct fuel injection systems may improve cylinder charge cooling so that engine cylinders may operate at higher compression ratios without incurring undesirable engine knock. Meanwhile, port fuel direct injection (PFDI) engines that include both port injection and direct injection of fuel may advantageously utilize each injection mode. For example, at higher engine loads, fuel may be injected into the engine using direct fuel injection for improved engine performance (e.g., by increasing available torque and fuel economy). At lower engine loads and during engine starting, fuel may be injected into the engine using port fuel injection to provide improved fuel vaporization for enhanced mixing and to reduce engine emissions. Further, port fuel injection may provide an improvement in fuel economy over direct injection at lower engine loads. Further still, noise, vibration, and harshness (NVH) may be reduced when operating with port injection of fuel. In addition, both port injectors and direct injectors may be operated together under some conditions to leverage advantages of both types of fuel delivery or in some instances, differing fuels.
DI engines and PFDI engines include a lift pump (also termed, low pressure pump) that supplies fuel from a fuel tank to a direct injection fuel pump (also termed, a high pressure pump) and, if present, a port injector fuel rail. The direct injection fuel pump may supply fuel at a higher pressure to direct injectors. During operation, one or more hot spots may be formed on a bottom surface of a pump piston within the direct injection fuel pump. As such, fuel may be exposed to the bottom surface of the pump piston when residing within or flowing through a chamber (herein termed a step chamber) formed underneath the bottom surface of the pump piston. Accordingly, fuel may be heated leading to fuel vaporization within the step room. Further, the evaporation of fuel may overheat the step room and may increase a likelihood of the pump piston seizing within a bore of the direct injection fuel pump.
The inventors herein have recognized the above-mentioned issues and identified an approach to at least partly address the above issues. In one example approach, a method for a direct injection fuel pump in an engine may comprise increasing a pressure in a step chamber of the direct injection fuel pump during at least a portion of a pump stroke in the direct injection fuel pump, the pressure increased to higher than an output pressure of a lift pump. Thus, formation of vapor in the step chamber may be reduced.
As an example, a direct injection fuel pump used in DI and/or PFDI engines may include a piston reciprocating in a bore, the piston being driven by a crankshaft in the engines. A compression chamber may be formed on a first side of the piston and a step chamber may be formed on a second side of the piston wherein the first side and the second side are positioned opposite each other. In one example, the compression chamber is formed vertically above a top surface of the pump piston while the step chamber is formed vertically underneath the bottom surface of the pump piston. To reduce fuel vaporization in the step chamber of the direct injection fuel pump, pressure in the step chamber may be increased at least during a portion of a pump stroke. The pump stroke may include either a suction stroke or a compression stroke. Pressure in the step chamber may be increased during the suction stroke by positioning a pressure relief valve upstream of an inlet to the step chamber. Pressure in the step chamber may be increased during the compression stroke by delivering fuel from the compression chamber to the step chamber.
In this way, pump degradation may be reduced. By increasing pressure in the step chamber during at least a part of each suction stroke and compression stroke in the direct injection fuel pump, fuel heating within the step chamber of the direct injection fuel pump may be reduced. Consequently, fuel vaporization within the step room may be diminished leading to enhanced DI fuel pump performance. Overall, durability of the direct injection fuel pump may be extended, and maintenance costs may be decreased.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.